The invention relates to a combined process for preparing diaryl carbonate and concentrating wastewater containing sodium chloride by osmotic distillation with simultaneous dilution of the sodium hydroxide solution obtained from the electrolysis for the diphenyl carbonate production process.
The preparation of diaryl carbonates (diphenyl carbonate) is usually carried out by means of a continuous process by preparation of phosgene and subsequent reaction of monophenols and phosgene in an inert solvent in the presence of alkali and a nitrogen catalyst in the interface.

The preparation of diaryl carbonates, e.g. by the phase interface process, is described in principle in the literature, e.g. in Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pp. 50/51.
U.S. Pat. No. 4,016,190 describes a process for preparing diaryl carbonates, which is operated at temperatures of >65° C. The pH is firstly set to a low value (pH 8 to 9) and subsequently a high value (10 to 11) in this process.
Optimization of the process by improving mixing and adherence to a narrow temperature and pH profile and isolation of the product are described in EP1219589 A1, EP1216981 A2, EP1216982 A2 and EP784048 A1.
However, a high residual phenol value in the wastewater from these known processes, which can pollute the environment and pose increased wastewater problems for the water treatment works, makes complicated purification operations necessary. Thus, WO 03/070639 A1 describes removal of the organic impurities in the wastewater by extraction with methylene chloride.
The solution containing sodium chloride is usually freed of solvents and organic residues and then disposed of.
According to EP 1200359 B1 (WO2000078682 A1) or U.S. Pat. No. 6,340,736, the wastewater containing sodium chloride can be purified by ozonolysis and then used in the electrolysis of sodium chloride. A disadvantage of this process is the very costly ozonolysis.
According to EP 541114 A2, a wastewater stream containing sodium chloride is evaporated to remove the water completely and the salt which remains together with the organic impurities is subjected to a thermal treatment, as a result of which the organic constituents are decomposed. Particular preference is given here to the use of infrared radiation. A disadvantage of the process is that the water has to be evaporated completely, so that the process cannot be carried out economically.
According to WO 03/70639 A1, the wastewater from DPC production is purified by extraction and then fed to the electrolysis of sodium chloride. However, only a maximum of 26% of the sodium chloride from the wastewater from DPC production can be recycled in the NaCl electrolysis since in the case of larger amounts of NaCl-containing wastewater the water introduced with the NaCl-containing wastewater into the electrolysis would upset the water balance of the sodium chloride electrolysis.
The solutions containing sodium chloride which are obtained in DPC production typically have a sodium chloride content of from 13 to 17% by weight. The entire sodium chloride present in the solutions can therefore never be recycled to the NaCl electrolysis to form chlorine and sodium hydroxide. At a sodium chloride concentration of 17% by weight in the standard sodium chloride electrolysis using a commercial ion-exchange membrane, which displays a water transport of 3.5 mol of water per mol of sodium, only about 23% of the sodium chloride from the solutions containing sodium chloride can be used. Increasing the concentration to about 25% by weight of a saturated sodium chloride solution would allow 38% of the sodium chloride present in the solution containing sodium chloride to be recycled. Recycling of all the solution containing sodium chloride is not known at present.
On the other hand, concentration processes by means of which water is withdrawn from the wastewater containing alkali metal chloride are known.
According to WO 01/38419, the solution containing sodium chloride can be evaporated by means of thermal processes so that a highly concentrated sodium chloride solution can be fed to the electrolysis cell. However, the evaporation is energy-intensive and costly.
It is also possible to use, for example, reverse osmosis or particularly preferably membrane distillation or membrane contactors (see MELIN; RAUTENBACH, Membran-verfahren; SPRINGER, BERLIN, 2003). A disadvantage here is the high energy consumption for overcoming the high osmotic pressures, as a result of which the process is no longer economical.
The abovementioned integrated processes all have the disadvantage that, in combination with a preparation of diaryl carbonate, it is possible to feed concentrated NaCl solutions (10-20% by weight) to the electrolysis only to a limited extent, so that NaCl can be reused only partially or increasing the concentration is energy-intensive and costly.
In view of the abovementioned prior art, it is an object of the invention to provide a process for preparing diaryl carbonate which gives products in high purity and good yield and enables a reduction in environmental pollution and wastewater problems in the water treatment works to be achieved by maximized recirculation of process wastewater solutions originating from diaryl carbonate production.
Furthermore, the conversion of sodium chloride into chlorine and sodium hydroxide and, if appropriate, hydrogen by electrolysis should be effected with minimal energy consumption and therefore in a resource-conserving manner in the recycling process.
The object is achieved by utilizing wastewater phases containing sodium chloride in the process by means of a preceding concentration increase of the NaCl solution from the preparation of diaryl carbonate for the electrolysis by means of an osmotic membrane distillation.
It has been found that the wastewater solutions containing sodium chloride which are obtained in the continuous preparation of diaryl carbonates by reaction of monophenols and phosgene in an inert solvent in the presence of alkali and a nitrogen catalyst in the phase interface can be concentrated directly, without complicated purification, in an osmotic membrane distillation after adjustment of the pH to a value of less than or equal to 8 and simple treatment with activated carbon and can be fed to an electrochemical oxidation of the sodium chloride present to chlorine, sodium hydroxide and, if appropriate, hydrogen, with the chlorine being able to be recycled at least partly for the preparation of phosgene.